Most of the interpretations aren’t “pure” in the sense of being completely independent of empirical constraints. Decoherence is one that posits a physical process (unlike many worlds and quantum Bayesianism) that doesn’t require new laws of physics (unlike objective collapse theories, including gravity mediated). It requires a collective effect from the environment, which is why I say it’s like thermodynamics.
However, it isn’t as well-understood as thermodynamics. I’ve read some papers in which the authors describe a simple, special-case example of an environment and show that a quantum system does get pushed toward basis states like 0.001% spin-up and 99.999% spin-down. In fact, part of the problem is, “What’s special about the basis states we see in the lab, like position, momentum, energy, spin up/down, field value, particle number, etc., rather than any other linear combination of them?” I remember reading about simple models in which an “isolated” particle, i.e. one surrounded by low-energy photons spontaneously coming out of the vacuum, is pushed toward its energy basis (think of a hydrogen atom, defined by its energy levels) and a colliding particle involved in one high-energy collision is pushed toward its position basis (a wave that becomes more particle-like on impact).
You’re reminding me that I found decoherence more compelling than the rest. But what’s missing, unless there’s been a breakthrough I don’t know about, is generalization. These were very simplified special cases. Also, there’s still a philosophical choice to be made because this world-view is taking the collapse seriously as an objective physical process, but not a discrete one that leaves us in exact basis states. It’s more like a phase transition that isn’t infinitely sharp.
I doubt that anything specific to the Standard Model (high-energy generality, like electroweak symmetry breaking) has anything to do with it, since low-energy electrons also need to decohere, but you probably meant, “standard physics, no new laws,” right? Decoherence is that: no new laws (unless there are yet more models of decoherence that I don’t know about).
I have heard of versions of many-worlds that are supposed to be testable, and you’re probably referring to one of them. The one that I’m most familiar with (“classic many-worlds”?) is much more of a pure interpretation, though: in that version, there is no collapse and the apparent collapse is a matter of perspective. A component of the wavefunction that I perceive as me sees the electron in the spin-down state, but in the big superposition, there’s another component like me but seeing the spin-up state. I can’t communicate with the other me (or “mes,” plural) because we’re just components of a big vector—we don’t interact.
On the other hand, classic decoherence posits that the wavefunction really does collapse, just not to 100% pure states. Although there’s technically a superposition of electrons and a superposition of mes, it’s heavily dominated by one component. Thus, the two interpretations, classic many-worlds and classic decoherence, are different interpretations.
If the state of theory and/or experiment developed further and these asymptotic projections were shown to exist in generality, that would bolster decoherence but not eliminate many-worlds: you could still imagine a 0.0001% me being coupled with the spin-up electron. It would, however, undermine the motivation. If, instead, these asymptotic projections were shown to not exist, then that would undermine decoherence to the point of refutation (i.e. only die-hards would keep looking for variants of the theory that aren’t ruled out). So classic decoherence is more falsifiable than classic many-worlds. That’s what I mean by saying that many-worlds is more purely philosophical.
But I was careful to say “classic” everywhere. With this being such an active area of research, I’m sure there are versions of these theories that don’t fit the description above. (They’re all “more than one theory.”) As I said, I’ve heard of variants of many-worlds that are less purely philosophical, and that must be what you’re referring to, @TAG.